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Dog stroking can transmit debilitating parasite

23 April 2003

By Andy Coghlan

People can become infected with a worm that causes blindness simply by stroking the coats of dogs that carry the parasite.

The finding challenges previous assumptions that the worms only spread to people who come into contact with dog faeces, and suggests that owners need to be extra vigilant in washing and worming their pets.

In the US, around 10,000 people a year, mainly children under 12, become infected with Toxocara canis, a nematode worm that grows up to 20 centimetres long in the intestines of dogs.

In Britain the risk seems to be lower&colon; “Fewer than 20 people a year get infected out of 60 million,” says Ian Wright, a vet practising in Burnham-on-Sea, Somerset, whose findings are reported in The Veterinary Record (vol 152, p 419). Also, a quarter of the population have antibodies to toxocara, having developed immunity through exposure.

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In infected people the worms can grow in the back of the eye, causing lesions which can interfere with vision or be mistaken for tumours. Occasionally they cause blindness. If they migrate to other organs such as the liver or lung they can also cause a form of toxocariasis, whose symptoms include periodic bouts of lethargy, similar to glandular fever.

Sticky eggs

Research by Wright and his co-investigator, Alan Wolfe, suggests that faeces might not be the only route of infection. Of 60 dogs they examined, a quarter had eggs of the worm in their hair. Three of the 71 eggs they recovered contained mature worm embryos which can infect humans. Embryos had begun to develop in 25 per cent of the eggs. They found up to 180 eggs in a gram of dog hair, a much higher density than is found in soil.

The vets say theirs is the first study to examine the coats of dogs as a route of infection. “We looked mainly at the anal region and the backs of the legs,” says Wright. The millimetre-long eggs are very sticky and could easily be picked by someone stroking the animal, he says.

The finding points to toxocara infection being a dog ownership problem, Wright says. “It has probably got very little to do with dogs fouling public places.” It also shows that dog owners should regularly wash and worm their pets, especially puppies, which are often infected through the placenta if the mother has worms.

Puppies should be wormed every fortnight for the first three months, then once a month for the following three months, says Wright. After six months, dogs develop some immunity and only need worming every six months and washing every fortnight. Wright’s most important advice is for people handling dogs&colon; “Wash your hands before meals, and after a good cuddle,” he says. And ensure good hygiene if infants and babies share the house with dogs.

A spokesman for the Chartered Institute of Environmental Health, which represents Britain’s environmental health inspectors, says the finding should be kept in proportion, as there are so few cases of toxocariasis a year. But he echoed Wright’s warnings about the importance of hygiene, both for dogs and for people who handle them.